Electrolytic cells for the generation of chemical reaction products are widely employed. One field particularly where these cells have found widespread use is in generation of halogens and caustic compounds from salts of the halogen. In such cells, the halogen is generally evolved at the anode, while the caustic compound is evolved adjacent the cathode.
Recently a considerable effort has been directed towards the development of improved anode configurations that enable operation of the electrochemical cell more efficiently. These efforts have born fruit in the development of such improvements as dimensionally stable anodes DSA.RTM. a proprietary anode coating system of Diamond Shamrock Corporation. Anode improvements have assisted in improving economics in operating chloralkali cells.
In cells where a separator such as a diaphragm separates the cell defining anode and cathode compartments, considerable effort has been devoted to development of improved separators. Separators based, for example, upon perfluorocarbon copolymers and having pendant cation exchange functional groups have been identified as providing, under certain cell operating conditions, the opportunity for achieving considerable economic advantage in operating a cell.
One remaining inefficiency in electrochemical cell operation is associated with power inefficiencies having their root in spacing imposed in most conventional cells between the separator and anodes and cathodes utilized in the cell. A variety of reasons can exist for the presence of the spacing. One common reason relates, for example, to gas bubble release difficulties where an electrode is pressed into a relatively soft separator such as a diaphragm type separator.
Spacing between anode and cathode in an electrochemical cell requires electrical current to follow a current pathway through cell electrolyte(s) where resistance to current passage can be relatively elevated. Generally, wider spacings between anode and cathode require that a more elevated voltage be applied to the cell to effect the desired electrochemical reaction. This elevated voltage requirement adds to electrical power consumption in operating the cell, adding to costs of cell operation.
A number of proposals exist focused upon reducing anode cathode spacing within a cell, and thereby reducing power consumption associated with cell operation. Reduced anode cathode spacing has been proposed for application to cells separated by a hydraulically permeable diaphragm and by a hydraulically impermeable membrane.
In diaphragm cells, for example, the spacing between anode and cathode has been reduced until one or both of the electrodes contacts the diaphragm. Many diaphragms are fabricated from materials which subject the diaphragm to swelling in cell environment. Electrodes utilized in such cells are frequently of a wire or mesh construction. Swelling of a diaphragm in contact with such an electrode can cause partial plugging of apertures in the electrode leading to poor release of gas bubbles being generated adjacent the electrode, and restricted flow of electrolyte from anode to cathode compartment through the diaphragm. One resulting repercussion can be an overvoltage at the electrode offsetting power gains achieved by reducing anode cathode spacing at least in part.
In membrane cells, the membrane, generally a cation exchange material, is normally quite thin, being on the order of a few mils. In addition such membranes frequently exhibit substantial dimensional stability, making placement of electrodes adjacent the membranes feasible without substantial risk of membrane expansion plugging apertures in the electrode. However when mesh electrodes or those fabricated from wire are placed adjacent a membrane allowing the electrodes to be within a few mils of one another, lines of electric flux between the individual elements of the electrodes do not always encompass all of the membrane material separating the anode and cathode resulting in inefficient use of the membrane and a corresponding increase in voltage drop attributable less than optimal electrolytic flux through the membrane.
Additionally, where a grid or mesh type electrode is contacted with a membrane, gas bubbles tend to agglomerate within apertures of the grid and these gas bubbles often lead to an overpotential at the electrode.
In one proposal for a closer anode to cathode spacing, as shown for example in U.S. Pat. Nos. 4,253,924; 4,253,922; and 4,250,013, a porous perhaps conductive secondary electrode material is utilized to fill, particularly the cathode compartments of the cell, and to press a primary electrode into contact with a cell membrane. An interface between the primary electrode and the porous secondary electrode material can substantially contribute to electrode resistance between the two and at least partially negate advantages otherwise available from the large electrode surface area potentially presented by the secondary electrode materials.
A cell configuration wherein a primary foam or reticulate electrode contacts a cell divider offers potential for improved economics in the operation of electrolyte.